One of the most common methods of storing digital data is to use a stack of one or more disks, mounted for simultaneous rotation about a common axis. Data are stored in concentric tracks about this axis on the surfaces of the disk or disks. The data are written onto and/or retrieved from these tracks by a transducing head, which can scan radially across the disk surface. By rotating the disk to the correct azimuthal angle, and moving the head to the correct radial distance, it is possible to access any point on the disk surface. The constant desire to increase the density of data storage on such disks means that the separation between adjacent tracks must be made as small as possible. This demands very accurate positioning of the head in order for the correct track to be located.
In one type of disk file, a stationary spindle shaft is rigidly fixed directly to a support structure. This support structure generally forms part of the walls of the disk file. The disks are mounted on a hub, which is caused to rotate about the spindle shaft by an electric motor within the hub. The heads are mounted on an actuator which produces their desired radial movement. This can be achieved either by rotating the actuator about a stationary shaft parallel to the spindle shaft, so that the head pivots in an arc across the disk surface, or else by linear motion of the actuator so that the head moves across the disk surface towards and away from the disk centre, effectively in a purely radial direction. The actuator is also mounted on the support structure: in the case of a rotary actuator, via an actuator shaft.
Often the spindle shaft is made of steel, which improves the efficiency of the in-hub motor, whilst the support structure is aluminium for lightness. Disk files of this general design are described in EP-0222939-A1 and U.S. Pat. No. 4,797,762.
For correct operation of the disk file, it is essential that each head can be accurately positioned over the desired track on the disk surface. This means that any uncontrolled movement of the head and the disk surface must be minimised, implying that the mountings and supports for the disk and the head must be as secure as possible. Usually the support structure comprises at least one piece of cast metal, the cast metal imparting mechanical strength and structural rigidity to the device. Note that in the type of disk file described above, the materials of the shaft and its support structure may be different, causing a potential problem with differential thermal expansion. Careful design of the means of attachment of the spindle shaft and actuator to the support structure is necessary to minimise unwanted movement of the disk and head.
U.S. Pat. No. 4,797,762 discloses one method of securing a shaft to its support structure in a disk file. The bottom of the spindle shaft is firmly glued into a hole in a first support structure, whilst the top of the spindle shaft is attached to a second support structure using a shouldered washer or bush. The shaft is bolted to the washer and the washer glued to the outer casing. The hole for the washer in the second support structure is slightly larger than the washer itself, allowing a certain leeway lest the holes for the top and bottom of the shaft are not quite properly aligned. By allowing this freedom in positioning of the washer, the washer can be glued into place to retain the shaft end in a stress-free manner. In this device, in common with many others, the top and bottom of the shaft are attached to two separate support structures. Such a design is susceptible to misalignment of the two shaft holes, and may also not hold the shaft as rigidly as desired.
The stability of the disk file can be improved if both ends of the shaft are attached to the same support structure. This is the case in EP-0222939-A1, in which a spindle shaft is attached at each end to two flanges of a common support structure by a bolt perpendicular to the axis of the shaft that passes into the thickness of the disk file wall.
This means of attachment has the disadvantage that it can only be used for certain forms of support structure, and it also requires the disk file wall to be thick enough to accommodate the bolt, which increases the weight and decreases the storage capacity of the disk file.
An alternative design, described in pending PCT application PCT/GB89/00267, is to attach the spindle shaft at both ends to holes in a single support structure. For assembly reasons, it is necessary for the shaft to be no longer than the interior wall separation that it spans, making it difficult for any portion of the shaft to protrude into the holes in the support structure. In this case, the shaft can be clamped to the support structure, allowing for a thinner and lighter support structure than in EP-0222939-A1. In PCT/GB89/00267, the spindle shaft is clamped to the support structure by bolts that pass through the support structure wall and down the shaft axis, with the clamping force being transferred to the shaft end by the support structure.
Clamping the shaft to the support structure in this manner however, can increase problems with thermal distortion if the shaft and the support structure have different coefficients of thermal expansion. To keep the central axis of the shaft stationary with respect to the support structure as the temperature changes, the shaft end and the inner surface of the support structure should slip past one another in the radial direction. In practice however, there is a tendency for the surfaces to lock together at their point of contact where friction is greatest. It is this point, rather than the shaft axis, that remains stationary. Both the shaft axis and support structure will then expand away from this point, producing a lateral shift between them, dependent on their respective coefficients of thermal expansion. Because the locking point is generally determined by manufacturing irregularities, and may vary due to wear during the lifetime of the disk file, this shift between the support structure and the shaft axis is essentially unpredictable. This makes it especially difficult for the servo system to compensate for any head/disk misalignment produced by the resultant tilting of the spindle or actuator, particularly where a dedicated servo system is used.
The problem of differential expansion is therefore particularly acute when the primary contact between the shaft and the support structure at both ends is in a plane perpendicular to the spindle shaft axis. This is not the case in U.S. Pat. No. 4,797,762, because the spindle shaft is firmly located at the end where it protrudes into and is bonded to the main support structure.